docs: def setPrefilter(self, edfsignal, prefilter):
          """
          Sets the prefilter of signal edfsignal ("HP:0.1Hz", "LP:75Hz N:50Hz", etc.)
  
          :param edfsignal: int
          :param prefilter: str
  
          Notes
          -----
          This function is optional for every signal and can be called only after opening a file in writemode and before the first sample write action.
          """
          if edfsignal < 0 or edfsignal > self.n_channels:
              raise ChannelDoesNotExist(edfsignal)
          self.channels[edfsignal]['prefilter'] = prefilter
          self.update_header()